The invention relates to rubber compositions with increased shelf life and reduced cure temperatures and times. More particularly, it relates to the use of titanium and zirconium compounds to produce rubber compositions that remain moldable for a long period of time in an uncured state, but that quickly cure on application of heat.
Synthetic and natural rubbers have a variety of unique and useful physical properties. In an uncured or natural state, such materials generally exhibit properties that are less than optimal for everyday or industrial use. Accordingly, rubber compositions are generally reacted with crosslinkers such as those containing sulfur or peroxide in order to cure the rubber to produce industrial articles having acceptable properties. In general, heat is applied to a rubber composition during a molding operation to produce molded articles having desired physical properties.
For many reasons, it is often desirable to increase the rate of cure in such molding operations. If the cure rate can be increased, articles can be molded for shorter times or they can be molded for the same time at lower temperatures. In either instance, a process is generally cheaper if the rate of cure is faster. Over the years, a variety of additives has been developed that can be added to rubber compositions to generally increase the cure rate. Examples of such additives include the well-known sulfenamide accelerators. Using the known accelerators, rubber compositions can be formulated having a wide range of cure rates. In general, it would be desirable to provide rubber compositions having even greater cure rates so as to achieve the benefits noted above.
Natural and synthetic rubber in an uncured state is subject to deterioration by oxidation because of the reactive double bonds in the molecule. U.S. Pat. No. 4,214,058 to Imamura et al. discloses the use of a narrow class of titanium organic complexes to reduce the susceptibility of such unvulcanized rubber to oxidative deterioration during storage. In addition to oxidative deterioration during storage, rubber compositions are subject to a number of other processes during storage that tend to reduce the useable life of the uncured composition. Thus, rubber compositions containing both curing agents and the usual accelerators have a limited shelf life because of a finite rate of cure even at room temperature. It has been generally observed that a formulated rubber composition containing curing agents can be reliably molded into useful articles for only a few days after compounding even if it is stored at room temperature. The useful xe2x80x9cshelf lifexe2x80x9d might be extended by refrigerating the uncured rubber compositions, but this would generally result in too much added expense. It is believed that the short shelf life of the uncured rubber compositions is due to the fact that curing, which is a chemical reaction, takes place at a finite rate even at room temperature. In fact, the accelerators used to increase the cure rate at the higher temperatures typical of the molding operation also tend to increase the cure rate at room temperature. For this reason, the uncured rubber compositions gradually cure at lower temperatures until they can no longer be molded successfully. Compounding the problem is the general observation that rubber articles cured at low temperatures such as room temperature and even up to about 120xc2x0 F. or slightly above tend to have less than desirable physical properties. As a consequence, rubber compositions that have been in storage at room temperature for a number of days become useless for producing industrial articles.
As a consequence, rubber compositions must as a rule be molded into a finished article within a few days of formulation. This is a disadvantage when for logistical or other reasons it would be desirable to formulate a rubber composition and hold it for a period of time before molding. For example, it may be desirable to compound a composition in a central location and carry out molding at a number of satellite sites distant from the central site. Such may be the situation for example, when a number of manufacturing sites are located far from one another, perhaps even in different countries. It would then be desirable to save the capital expense of providing each molding facility with expensive compounding and storage facilities. In such a situation, it would be advantageous to compound the composition centrally and ship it to remote sites in a just-in-time fashion for molding. In such a situation, quality control of the manufactured articles would also be simplified because all compounding could take place in one or only a few locations.
As noted above, additives of the prior art that increase the rate of cure under molding conditions also tend to decrease the shelf life by also increasing the rate of cure at room temperature. It would thus be desirable to provide rubber compositions that have desirably fast cure times at higher temperatures, but which nevertheless maintain a shelf life at lower temperatures such as room temperature. Preferably, such compositions would provide for increased rates at temperatures in the range of 100xc2x0 C. 200xc2x0 C., but which slow down the rate of self cure upon standing at room temperature.
It has been surprisingly found that a group of titanium and zirconium compounds with specially defined structures are useful for preparing rubber compositions with such cure properties. Although similar compounds have been used in rubber compositions of the prior art, the specific structures of the invention have not been shown to provide rubber compositions with enhanced cure rates at elevated temperatures, improved cure rates at temperatures lower than typically employed, and increased shelf life at room temperature.
Rubber compositions contain, in addition to synthetic or natural rubber and conventional curing agents, an auxiliary composition containing a titanium or zirconium compound. The compositions are characterized by an enhanced cure rate at elevated temperatures such as would be useful for producing molded articles. At the same time the compositions exhibit an increased shelf life whereby uncured compositions of the invention are stable and can be molded for a considerable period of time even after storage at room temperature. Titanium compounds of the invention have at least one alkoxy group bonded to titanium, and zirconium compounds of the invention have at least one alkoxy group bonded to zirconium. In preferred embodiments, the auxiliary composition contains chelates of the titanium or zirconium compound. The auxiliary composition works to provide the rubber compositions of the invention with curing behavior which was not to be expected from conventional accelerators of the prior art. In particular, the auxiliary composition enhances the rate of cure at elevated temperature, and retards curing at room temperature.